1. Technical Field
The present invention relates to a virtual image display apparatus which provides video formed by an image display element or the like to an observer, and in particular, to a virtual image display apparatus which is suitable for a head-mounted display worn by the head of the observer.
2. Related Art
Various optical systems have been proposed as an optical system which is assembled in a virtual image display apparatus, such as a head-mounted display (hereinafter, also referred to as an HMD) worn by the head of an observer (see JP-A-9-197336, JP-A-9-73043, Japanese Patent No. 4069814, JP-A-2002-90690, and JP-A-2004-341324).
For the virtual image display apparatus, such as an HMD, it is desirable to enlarge a viewing angle of video light and to reduce the weight of the apparatus. In particular, in order to improve the fit, it is important to reduce the thickness in a visual axis direction of the observer and to bring the center of gravity close to the observer.
Since it is advantageous that an optical path is bent using a reflection surface for miniaturizing an optical system and reducing the weight thereof, various optical systems using a prism have been developed. At this time, it is important to use a reflection surface or a refractive surface as a non-axisymmetric curved surface and to effectively perform aberration correction by applying the degree of freedom of the shape thereof.
For example, in JP-A-9-197336 and JP-A-9-73043, an optical system in which reflection is performed four times on the inside of a prism is disclosed. This type of optical system has an extremely simple configuration and is advantageous for weight reduction, but reflection is made two times on a surface closest to eyes of an observer so that the degree of freedom of the shape is small and optical performance is insufficient.
In Japanese Patent No. 4069814, an optical system in which reflection is made four times in the inside of a prism is disclosed, but the first, second, third, and fourth reflections are respectively made on the same reflection surface so that the degree of freedom of a non-axisymmetric surface is not utilized and aberration correction is insufficient.
Further, in JP-A-2002-90690 and JP-A-2004-341324, an optical system in which reflection is made four times in the inside of a prism is disclosed, but a relay optical system cooperatively forming an intermediate image is formed. Therefore, an entire optical path is long and a thick prism is necessary to be used so that the weight thereof is increased.
In addition, in a case of an optical system of forming an intermediate image, it is possible to generate an image with a wide visual angle even when an element allowing an image of a video element or the like to be generated is arranged in a position separated to some extent from the position of eyes of an observer. However, it is likely that the optical design becomes complicated, and dust, bubbles, and the like are present in the vicinity of an intermediate image, or when discoloration or the like is locally generated in an optical member due to deterioration with time, there is a possibility that these become enlarged and the display quality is deteriorated.